Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
In Ex. 2:3, 5, Isa. 19:6, it is the rendering of the Hebrew suph, a word which occurs frequently in connection with yam; as yam suph, to denote the "Red Sea" (q.v.) or the sea of weeds (as this word is rendered, Jonah 2:5). It denotes some kind of sedge or reed which grows in marshy places.